The heat exchange surfaces in the heat exchange section of the boiler plant generally comprise at least superheater tubes of different superheating stages and an economizer. The temperature of water or steam is gradually raised in these different heat exchange surfaces. In order to achieve an end temperature, which is as high as possible, different heat exchange surfaces are most advantageously arranged to a heat exchange section in an order according to their desired end temperatures in such a way that hot flue gas entering from the combustion section encounters first the heat exchange surface which has the highest end temperature.
The heat exchange surfaces are usually packages of mainly horizontal, several times back and forth, bent tubes. Generally, the diameters of the tubes and the distances between the tubes diminish when moving towards lower temperatures. For example, the distances between the economizer tubes from each other are usually shorter than the distances between the tubes of the superheater packages, in order to achieve a sufficient heat exchange efficiency.
Ashes and other particles which can stick on the heat exchange surfaces are entrained with the flue gases exiting the combustion chamber. The thus generating deposits diminish the heat exchange coefficient, and thus, also diminish the heat exchange efficiency. Thick deposits may also disturb the flow of the gas between the heat exchange tubes, which further diminish the heat exchange efficiency.
When the ash deposit layer becomes too thick, it may fall off by itself from the surface of the heat exchange tubes, or it may be dislodged by some appropriate sootblowing method. The dislodged ash drops back to the combustion section or to the bottom of the heat exchange section, where it may be guided, for example, to the ash discharge system of the plant.
In order to avoid the sticking of falling ashes on the lower heat exchange surfaces, a so-called tower boiler construction is preferably used, especially when combusting fuels with difficult ashes, such as brown coal. The heat exchange section in the tower boiler is arranged above the combustion section in such a way that the flow direction of the flue gas is in the heat exchange section from the bottom upwards. Thus, the ashes that fall off or are dislodged from the upper heat exchange surfaces of the heat exchange section fall towards the sparse tube packages lower, and the risk of falling ashes sticking on the lower packages is rather small.
Large boiler constructions are usually built to be top-supported in such a way that a stationary supporting construction is set up around the boiler, and the boiler is assembled to hang on strong suspension wires from a supporting plane at the upper part of the support construction. Problems with a tower boiler built as indicated above relate to the fact that the height of a one-piece construction becomes very high. In fact, the height of a tower boiler in a large power plant may be more than 100 m.
When the boiler is started up, the temperature of the boiler body rises by hundreds of degrees. The upper part of the boiler hung from above remains stationary, but the lower part thereof lowers down tens of centimeters. Such a great thermal motion sets very high flexibility requirements for the connections to be made to the lower part of the boiler tower. For example, the inlet systems for fuel and the outlet systems for bottom ash must be able to allow such a motion between the stationary parts of the system and the parts connected to the boiler.
The assembly of the boiler usually takes place gradually in such a way that the upmost parts are connected to the stationary supporting structure first. Only after the assembly of the upper parts, is it possible to connect the lower parts connecting thereto. Since a tower boiler is usually a very high, one-piece structure, the assembly thereof is very time-consuming. This slows down the construction of the boiler plant and increases the construction costs.